The present invention generally relates to laser instruments and, more particularly, to a bifocal lens assembly adapted for use in a laser scanner system which, on the same optical axis, transmits light to a spaced target, such as a product carrying a bar code symbol, and detects light reflected from the target.
Presently, bar code symbols are used on a broad range of retail products for check-out and inventory purposes and a variety of laser scanner systems have been developed to read such symbols. Both handheld and check-out counter-mounted scanner systems are widely used at the present time. In one version of such scanner systems, a retro-directive type optical system is employed in which a transmitted laser light beam is directed out along a path and the reflected light is collected back along the same path. Both bifocal and trifocal lens assemblies have been incorporated in the optical systems of such scanner systems.
One known bifocal lens assembly includes a large convex collection lens within which is formed a much smaller convex lens. The smaller lens is ordinarily positioned above the axis of the larger collection lens. In a retro-directive optical system employing such a bifocal lens, the laser beam is sent from the source laser through the smaller convex lens and then through scanning optics to the bar code target. The light reflected from the target is collected and received, in a much larger volume, through the large collection lens and focused and directed to a photodetector. Thus, the laser beam is originally transmitted through the smaller lens not along the optical axis of the larger lens, while the reflected and collected light passes back through the larger lens along its optical axis.
One known trifocal lens assembly of the above cross-referenced application includes a large collection lens having a first curved surface on the side which receives light reflected from a target and a second curved surface on the opposite side which faces a photodetector. Also, a small prism with a beam expander element thereon are integrally attached to and protrude outwardly from the second surface of the large collection lens, whereas a small focusing lens is formed on the first surface of lens. The small prism and beam expander element and the optical axis of the focusing lens are respectively disposed along and coaxial with the optical axis of the first and second surfaces of the large collection lens.
The beam expander element of the trifocal lens assembly first receives the laser beam at an angle inclined to the optical axis of the collection lens, and expands the beam. The expanded beam is reflected by the prism and routed along the collection lens optical axis to the focusing lens. The focusing lens converges the expanded beam and focuses it at the target via intervening scanning optics. A substantial volume of light reflected from the target is received back, via intervening scanning optics again, by the collection lens which images and transmits the collected light along the optical axis of the lens to a photodetector.
The challenge of the approach embodied in the above-described trifocal lens assembly is to somehow place on a common optical axis the components thereof which pass both transmitted light and reflected light without creating interference between the two. Although the prism and the beam expander element of the trifocal lens assembly protrude from the collection lens and are instrumental in routing the laser beam onto and along the optical axis of the collection lens, they are small enough in size so as to substantially minimize shadowing or obstruction of the reflected image.
Notwithstanding the fact that the trifocal lens assembly of the cross-referenced application is generally successful at accomplishing the objective of minimizing shadowing or obstruction of the reflected image, the prism and beam expander element thereof still tend to obscure a greater amount of the reflected light than is desirable. To obtain internal reflection within the prism of the laser beam, the angle between the beam and the optical axis of the lens must be rather large. This being the case, a somewhat elongated and enlarged prism element is required, which under environmental extremes causes the transmitted beam to be altered.
Consequently, a need exists for improvements in the arrangement and construction of the lens assembly so as to further reduce the degree to which the reflected image is obstructed and the transmitted beam altered by the protruding prism arrangement.